Not applicable.
Not applicable.
1. Field of Invention
The present invention is a method for estimating fluid pressures in a series of subterranean formations from formation interval velocity data. The invention measures directly, from the velocity data, the variations in the effective vertical stress, which are due to variations in the fluid pressures in the subterranean formations. The invention uses these variations to estimate the effective vertical stresses that, together with an estimate of the corresponding overburden stresses, allow estimation of the corresponding fluid pressures in the formations.
2. Related Art
Typically, while drilling an oil or gas well, the density of the drilling mud must be controlled so that its hydrostatic pressure is not less than the fluid pressure present in the pores in any formation along the uncased borehole (where the borehole is open to the formations). Otherwise, formation fluid may flow into the borehole. This can lead to a blowout if the flow is not stopped before the formation fluid reaches the top of the well. If the fluid contains hydrocarbons, this can result in fire or explosion.
Blowouts result where the mud weight is too low to balance fluid pressures within the subterranean formations. Excessive overbalance, where the hydrostatic pressure of the drilling mud greatly exceeds the fluid pressures in the subterranean formations, can induce undesirable fractures in the borehole wall that causes loss of drilling fluid. The drill pipe may also get stuck along contact zones with the borehole wall if the hydrostatic pressure of the mud is too much in excess of the fluid pressure in the subterranean formations. Also overbalanced mud typically reduces the penetration rate of the drill bit resulting in increases in drilling time which result in increased drilling costs. Therefore, to optimize drilling performance and minimize drilling problems, the mud weight must be adjusted according to the variation of the fluid pressures in the formations along the borehole. Prediction of these variations in the fluid pressure in the formations along the borehole is essential to safe and economical drilling.
The prior art contains numerous discussions of the problem and of the geologic factors involved that create anomalous fluid pressures within the subterranean formations. Anomalous fluid pressures can be attributed to several causes.
The basic physics of determining pore pressure is described by Terzaghi""s principle. Terzaghi""s principle states that the total downward force on an element of volume of rock is supported by two upward forces, the effective vertical stress (the part supported by the rock matrix) and by the fluid pressure of the fluid in the pore space of the rock in said element of volume. Terzaghi""s principal is expressed in equation form as:
S=P+EVS
where S=the downward force due to the weight of the overburden rock column,
P=the formation fluid pressure in the pore spaces of the rock, and
EVS=the effective vertical stress exerted upward by the rock matrix itself
Anomalous formation pressure can be caused by xe2x80x9cundercompactionxe2x80x9d, thermal expansion of the formation fluid trapped in the pore spaces of the formation (aquathennal pressuring), clay diagensis (expulsion/expansion of integranular water due to temperature changes) and various other causes. Undercompaction occurs when low permeability inhibits fluid in the pores of the formation from escaping as rapidly as the pore space would like to compact due to the force exerted by the weight of the column of rock above the formation.
The prior art contains numerous methods of computing the effective vertical stresses in the formations penetrated by the borehole, and subsequently computing the fluid pressure in those formations by subtracting the effective vertical stress, EVS, from the overburden stress, S. The prior art also contains numerous methods of computing pore pressure directly without having to compute effective vertical stress.
Most of the prior art techniques rely on either empirically derived baselines or empirically derived virgin curves in the form of interval velocity vs. depth. At least one prior art method expresses effective vertical stress in terms of temperature, age, rate of deposition, and other geologic parameters.
The technique described herein differs from previous methods and is useful because it can be employed, without detailed knowledge of the geological parameters at depth, to produce relative estimates of effective vertical stresses, and thus pore pressures, on an areal extent to the accuracy permitted by the input velocity functions. It is based upon the established fact that as the pore pressure changes in a given formation, the effective vertical stress changes, and thus the interval velocity changes in the formation. The pore pressure changes are thus reflected in the second derivatives of the velocity function changing as the second derivatives of the effective stress function changes at a given location.
The interval velocity information of the subterranean formations is usually obtained, at a specific location, from the sonic log obtained by passing a tool down the borehole and recording the interval transit times of sound passing through a given formation between a source and receiver within the tool. The interval velocities can then be directly calculated from the interval transit times. Also, the interval velocities of the subterranean formations can be obtained, both at the location to be drilled and over an areal map extent on a spatially varying basis by calculations on seismically recorded data.
This invention is unique in that it requires no baselines, virgin curves, nor detailed knowledge of geologic parameters at depth to compute, from the interval velocity vs. depth (or interval velocity vs. time) function, the effective vertical stress function. The only parameters needed are the water depth (for marine cases) and the initial conditions for at least the first depth interval of the effective vertical stress vs. depth to be computed plus the second derivatives of the interval velocity vs. depth friction. The first initial condition is that the effective vertical stress is zero at the mudline (for marine cases) or is zero at the surface for land cases. This is known to be true by definition. The second initial condition is to assume a rate of increase of effective vertical stress (a first derivative value) across the first depth (or time interval). For the depth case this first derivative is typically, but not limited to, the range of 0.465 psi/ft to 0.535 psi/ft. If the first interval is hydrostatically pressured (is in hydraulic communication with the surface) then theoretically this first derivative should be very close to 0.535 psi/ft (or the equivalent in time for the time case).
Generally sonic logs do not have sufficient information on the shallow data to compute the initial conditions directly from the data, the logs usually being started, especially in deep water, several thousand feet below the mud line. However, the newer generation of seismic 3D information usually provides rather good interval velocity information on the shallow zone below the mudline, and, when this is so, it is feasible to compute the initial conditions directly from the velocity data.
Since the second derivatives of the interval velocity function should be the same as the second derivatives of the desired effective vertical stress function, mathematics implies that an alternate solution using the first derivatives should be equally valid. This solution, in practicality, requires an accurate computation of the difference between the initial first derivative of the interval velocity function and the assumed first derivative of the effective vertical stress function as well as a knowledge, for marine cases, of the exact sediment velocity of the rock at the mudline.
The complete effective vertical stress vs. depth (or time) function can then be computed by numerical integration using these initial conditions and using the said second derivatives starting at the end of the initial interval. This is made possible by recognizing that the second derivatives of an effective vertical stress function will be the same, except for an initial interval, as those of the interval velocity function. This is the basis for this invention and this premise has been tested and found to be true. The resulting effective vertical stress functionsxe2x80x2 accuracies primary limitations are the accuracies of the velocity function and the choice of an original condition.
Because this invention is an effective vertical stress technique, Terzaghi""s principle holds and the computed effective vertical stress at each depth must be subtracted from the total vertical stress value corresponding to that depth to obtain the fluid pressure of the subterranean formation at that depth at the location where the interval velocity function applies. Accurate estimation of the total vertical stress (overburden stress) is necessary for final estimation of fluid pressures in the formations. The estimation of the total vertical stress relies on good empirical information about the densities of the area of interest and the relationship between these densities and seismic interval velocities.
Accordingly, the objectives of this invention are to provide, inter alia, a new and improved method of estimating downhole fluid pressures that:
effectively estimates and predicts pore pressure changes;
utilizes existing geological data; and
requires no geophysical baselines, virgin curves or detailed geologic parameters at depth.
These objectives are addressed by the structure and use of the inventive process.
The inventive method achieves improved accuracy in estimating pore fluid pressures in subterranean formations. It is based on the fact that the primary cause of changes in the interval velocities of rocks are thought to be due to changes in the effective vertical stresses, and the effective vertical stresses vary under the influence of:
(1) the effects of compaction;
(2) the effects of changes in rock type; and
(3) the effects of changes of fluid pressures within the pore spaces of the rock.
All of these effects contribute to changes in the effective vertical stress within a given rock volume and thus must be embodied in a function of interval velocity vs. depth (or time). Therefore the second derivatives of this interval velocity function must be reflected in the second derivatives of the effective vertical stress function vs. depth (or time).
An exception to this is thought to be the case of a formation heavily saturated with natural gas. It is well known that formations of this type exhibit abnormally low interval velocities without significant decrease in density. Examples have been observed where such rocks would cause the effective vertical stress to be underestimated by this technique and thus the pore pressures would be overestimated.
The initial conditions, for rocks normally pressured just below the surface (or mudline) are well known within narrow bounds and discussed well in the literature.
Any changes in the characteristics of the volume of rock under consideration do not influence the overburden stress due to the above column of rock, and given good density measurements, the fluid pressure in the pore space of the volume under consideration can be computed accurately by Terzaghi""s principle, S=P+EVS which holds true at the interface at the top of the volume.
This invention is thus an improvement to the problem of computing accurate effective vertical stress functions, previously based on empirical baselines and often insufficient data, and provides a method having improved accuracy and efficiency.
Other objects of the invention will become apparent from time to time throughout the specification hereinafter disclosed.